CONCHYLIA— D7THrR^. xvii. 



52. DisciNA. — Testa discoidea, valvd superiori medio mamillato- 

 productd. 

 Shell disk-shaped, with the upper valve protube- 

 rant in the middle. 



46. Pecten. — Testa basi auriculatd, fovea ca-dinali trigond. 



Shell auricled at the base, with a triangular hollow 

 at the hinge. 



45. OsTREA. — Testa basi inauriculatd, fovea cardinali trigond. 



Shell without auricles at the base, with a trian- 

 gular hollow at the hinge. 



** Cardo dentibus primariis, lateralibus nullis. 



Hinge with primary teeth, but no lateral ones. 



8. Mya. — Testa aquivalvis, dente eJevato transverso in valvd sin- 



istrd, dextrd muticd. 

 Shell equivalve, with an elevated transverse tooth 

 in the left valve, and none in the right. 

 10. Sphenia. — Testa ineequivalvis, dente elevato transverso in valvd 



sinistrd, dente concavo in valvd dextrd. 

 Shell inequivalve, with an elevated transverse tooth 

 in the left valve, and a concave tooth in the right. 



9. Lyonsia. — Testa incequivalvis, dente mobili comrnuni valvas 



coadunante foveold utrinqu^ obliqud. 

 Shell inequivalve, with a common moveable tooth 

 uniting the valves together by an obli(jue cavity in 

 each; 



